The process of growth (mass accumulation) is a critical determinant of cell, organ, and body size and is often deregulated in diseases such as cancer and diabetes. We are studying the TOR (mTOR in mammals; dTOR in drosophila) pathway, a conserved signaling system that is emerging as the critical regulator of growth in eukaryotes and is regulated by metabolism, growth factors and stress. The TOR pathway is the target of the FDA-approved immuno suppressant rapamycin that is also used to prevent vessel restenosis after angioplasty and is in trials for the treatment of cancer and autoimmune diseases. Over the last few years we have been studying the biochemistry of the mTOR pathway in human tissue culture cells. Although this work has been fruitful and has lead to the discovery of several proteins that interact with mTOR (e.g. raptor and GbetaL), we have come to realize that many of the important questions about TOR signaling are more easily answered in a model system that uses drosophila rather than human tissue culture cells. The human and drosophila TOR (dTOR) pathways are highly conserved and our preliminary data suggests that both pathways respond alike to nutrient metabolism, contain an unidentified TOR-regulated phosphatase, and have similar effects on cell size. In contrast, the yeast TOR pathway is missing several important components, including S6 kinase (S6K), TSC1, and TSC2, senses different nutrients and does not have growth factor inputs. There are several advantages to studying TOR signaling in drosophila rather than human tissue culture cells. The fly pathway has less redundancy than the human version, loss of function mutations are remarkably easy and efficient to make, and we have developed an ultra high-throughput technology (RNAi-cell microarrays) for undertaking genome-scale RNAi screens in drosophila cells. To exploit the potential of the drosophila system to study TOR signaling we propose to: (1) identify and characterize the metabolism-regulated mechanisms that control the TOR pathway; (2) identify and understand the regulation of a TOR controlled phosphatase that inhibits TOR effectors; and (3) use RNAi-cell microarrays to undertake large scale loss of function screens to discover new components of the TOR pathway. In general, we will begin our experiments in the drosophila system and, as our knowledge of a particular problem increases, extend our work to human cells and proteins. Our research will not only lead to a fundamental advance in our understanding of the mechanisms that regulate growth in eukaryotes, but also to the discovery of novel signaling mechanisms that will likely be of value as drug development targets.